In my opinion column this month, we are in search of mythical beings. No, not a veterinary surgeon looking for work, but a mythical artery: the big artery at the back of the eye, the one that runs in the optic nerve. The one for which you can buy a clamp from reputable instrument suppliers: enucleation forceps described online as “designed to slide around the rear of the globe and capture the blood vessels”.
At a relatively recent small animal ophthalmic CPD course – during those halcyon days of sitting on a plastic chair all day surrounded by living, breathing people – the lecturer asked the audience: “Who clamps and ties off the artery when doing an enucleation; who manages to tie it off?” Around half of the audience shot their hands up as they were keen to show they did what they should, while the others shuffled around a bit and looked guilty. A few of us were in on the joke and eagerly awaited the punchline. “Well, I’ve got news for you all,” the lecturer informed us, “it doesn’t exist. It does in humans, but not in dogs or cats.” At this, my colleagues who were rather guiltily internally reviewing their last surgical bloodbath looked slightly less ashamed.
This mythical artery is the canine, feline and equine equivalent of the human central retinal artery, the big one that runs in the optic nerve. So, why is the belief that we have to tie off this great big bleeder at the back of the eye so prevalent? Why can we physically buy an instrument to deal with something that does not exist? I cannot buy a unicorn dehorning instrument, so why this clamp? Why were some of us taught by other vets to encircle the back of the eye with Vicryl? Where did this myth come from?
We are very lucky to have a great library in our practice: a client arrived at the desk one day, years ago, and said that a friend of his was a vet. He had just died and he had some books; would we like them? He had a small box of slightly out-of-date texts so we politely accepted. An hour later he came back with the rest: a fantastic collection dating from 1896 to about 1980, which was around the time our own collection started. This means that I can do a literature review covering around 135 years to see where the myth came from.
First off is Mӧller’s Operative Veterinary Surgery, published in 1895. This is available as a reprint and I would advise putting it on your Christmas list. It covers everything from battlefield wound treatment in horses to stuffing a turnip up a cow’s backside for bloat but, sadly, no mention of the eye at all in that book.
Moving on to the Encyclopaedia of Veterinary Medicine, Surgery and Obstetrics (Wooldridge, 1923), the main discussion on eye disease in dogs centres on distemper. For corneal ulcers it suggests using zinc sulphate or boric acid; if the eye is irritated try a little belladonna; or for corneal opacity “… a little calomel may be blown into the eye once or twice a day”. The volume also offers a fantastic glimpse into the veterinary world they lived in: “… on the other hand, numerous agents have been placed on the market that are absolutely worthless and some have even proven dangerous”. This was 1923, so nothing really changes: the internet, Facebook, 1920’s pamphlets… the same story retold a hundred years on!
It is in 1960, in Clinical Veterinary Surgery by Shuttleworth and Smythe, that we first see the mythical beast: “… a heavy haemorrhage occurs during the process due to cutting of the central artery to the retina”.
Then, in the second edition of the otherwise excellent Fundamentals of Veterinary Ophthalmology (Slatter, 1990), we read of “… the internal ophthalmic artery that enters the optic canal with the optic nerve”.
The mighty two-volume box set of Veterinary Ophthalmology (Gelatt et al., 2013) does not specifically mention the artery but it does suggest that something is lurking behind the eye when it advises: “… expose the optic nerve which is clamped…”
In the next edition of Fundamentals of Veterinary Ophthalmology (Slatter, 2001), the anatomy of the eye has been slightly altered, but it is erroneously suggested that “… a ligature may be placed around the nerve”. They do then refine their anatomic description, however, by stating “… encircling the associated long and short posterior ciliary vessels before entering the sclera” before further commenting “… the contribution from the internal carotid is small unlike the situation in primates”. There is then a long description of the blood supply to the eye and a diagram that looks correct, but in a 2001 textbook this diagram is copyright 1964! It just shows how easily errors can be perpetuated, repeated and recycled.
The most recent edition of the BSAVA Manual of Canine and Feline Ophthalmology is very limited on anatomy compared to earlier texts; however, their advice is now correct: “… exposure and ligation of the optic nerve is not recommended” (Gould and McLellan, 2014).
The last words I will present on this subject are from my notes, compiled during my attendance of another lecture at BSAVA, circa 2015, by Marian Matas Riera, a diplomate of the European College of Veterinary Ophthalmologists (ECVO) (so she probably knows her eye anatomy!): “Dogs, cats and horses do not have a blood vessel in the optic nerve, but humans do. There is no need to clamp any retrobulbar structure or vessel prior to the removal of the globe.” Also, as we know, pulling around in the optic nerve can affect the other eye via the optic chiasma, especially in cats… or is that also a myth…?
Has anyone ever seen a stump pyometra? Can Boxers have acepromazine (ACP), and does ACP cause fitting? If you roll a dog on its back under general anaesthesia, will it get a GDV? If you have any other ideas for myth busting, please get in touch!
